Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: Quantos itens diferentes aparecem na coluna de matrícula que se juntaram em 1931, 1949 1?
 ###Tabelas: CREATE TABLE completed_onshore_wind_farms (completed_onshore_wind_farms_id AUTO_INCREMENT PRIMARY KEY, Wind Farm VARCHAR, Capacity (MW) INT, Turbines INT, Turbine Vendor VARCHAR, Size (MW) VARCHAR, County VARCHAR) CREATE TABLE newly_named_dinosaurs (newly_named_dinosaurs_id AUTO_INCREMENT PRIMARY KEY, Name VARCHAR, Novelty VARCHAR, Status VARCHAR, Authors VARCHAR, Unit VARCHAR, Location VARCHAR) CREATE TABLE current_members (current_members_id AUTO_INCREMENT PRIMARY KEY, Institution VARCHAR, Location VARCHAR, Nickname VARCHAR, Founded INT, Type VARCHAR, Enrollment INT, Joined VARCHAR) CREATE TABLE club_information (club_information_id AUTO_INCREMENT PRIMARY KEY, Club VARCHAR, Season's Last Head Coach VARCHAR, City VARCHAR, Stadium VARCHAR, 2004-2005 season VARCHAR) CREATE TABLE bowling_averages_top_10 (bowling_averages_top_10_id AUTO_INCREMENT PRIMARY KEY, Player VARCHAR, Team VARCHAR, Matches INT, Overs VARCHAR, Wickets INT, Average VARCHAR, Economy VARCHAR, BBI VARCHAR, 4wi INT, 5wi INT)  
SELECT COUNT Enrollment FROM current_members WHERE Joined = 1931, 1949 1